Why glass frogs have see-through skin becomes clear in study
The mystery of why glass frogs have see-through skin has been solved, scientists say: the unusual feature is a type of camouflage.
Glass frogs are found in tropical Central and South America, and get their name from their skin.
However, the frogs are not truly transparent but translucent, with the skin on their backs typically a vivid green and their intestines and heart visible through their underbelly. This has led to a question that has kept scientists on the hop.
“If predators cannot see straight though the frogs, why do glass frogs have transparent skin at all, and not the opaque camouflaged patterns of other tree frog species?” said Dr James Barnett, a postdoctoral researcher at McMaster University, Canada, who coauthored the study.
Barnett and colleagues say they have cracked the conundrum. “The frog is always green to generally match leaves, but leaves will differ in their brightness,” said Barnett. The team say that while the colour of the frog’s body changes little against dark or light foliage, the legs are more translucent and hence shift in brightness, helping the amphibians to blend in.
“By having translucent legs and resting with the legs surrounding the body, the frog’s edge is transformed into a softer, less contrasting gradient from the leaf to the legs, and again from the legs to the body,” said Barnett, noting that this makes the frog’s outline less recognisable to predators.
Writing in the US journal Proceedings of the National Academy of Sciences, Barnett and colleagues report how they carried out three experiments.
In the first, they photographed 55 glass frogs both on leaves and on a white background and then used computer models to compare the colour of the frog in each scenario. “We found that the colour of the frogs’ bodies did not change much between backgrounds, but the legs did change significantly,” said Barnett, adding that the change was down to a shift in brightness, not hue.
The results were the same when they modelled how different species may see these frogs, including a snake, a bird and a human. “The camouflaging effect is interpreted in a similar manner between humans and the frogs’ natural predators,” said Barnett.
The team then produced computer-generated images of glass frogs with different patterns of translucency against leafy backgrounds. Twenty people were each presented with 125 such images and asked to point out the frog as quickly as possible. The team found participants were quicker to spot the frog when it was fully opaque compared with frogs with a natural pattern of translucency.
Finally, the team made 180 translucent and 180 opaque frogs out of gelatine and put them in vegetation in Ecuador, monitoring over the course of 72 hours whether the frogs were attacked by birds. Overall, 53 opaque and 24 translucent frogs were eaten during the experiment.
“Our study shows that being translucent does help glass frogs camouflage themselves from predators, but not necessarily in the way expected by comparison to fully transparent species,” said Barnett.
Prof Devi Stuart-Fox, an expert on animalcolour and behaviour at the University of Melbourne, who was not involved in the research, said: “This is a fascinating study because it shows yet another form of camouflage in animal – the sheer diversity of camouflage strategies in nature is truly remarkable.”
Stuart-Fox said all three of the experiments had limitations, but taken together the evidence was compelling that the frogs’ translucency is a form of camouflage.
“Interestingly, the legs are more translucent than the body, making the edges of the body harder to distinguish,” she said. “Predators form a search image for the shape of their prey, so masking the body’s outline is a very effective strategy to enhance camouflage.”